1. Field of the Invention
The present invention relates to an apparatus and a method of forming multi-color images, and more particularly, to an electrophotographic color image forming apparatus and method using a multi-pass method by which a multi-color image is formed by repeatedly exposing, developing, and transferring toner of different colors using a laser scanning unit (LSU) and a photoreceptor medium.
2. Description of the Related Art
In general, an electrophotographic color image forming apparatus forms a latent electrostatic image by scanning light onto a photoreceptor medium charged with a predetermined potential, develops the latent electrostatic image into a predetermined color toner image using a developer unit, and transfers and fixes the predetermined color toner image to a paper to form a color image. Colors of toner used in a color image forming apparatus are generally yellow (Y), magenta (M), cyan (C), and black (K). Thus, four developer units to develop toner of four colors are required.
The method of forming a color image includes a single-pass method performed using four LSUs and four photoreceptor media and a multi-pass method performed using an LSU and a photoreceptor medium.
FIG. 1 is a schematic view of a color image forming apparatus using a single pass method. Referring to FIG. 1, the color image forming apparatus includes photoreceptor drums 120C, 120M, 120Y, and 120K, LSUs 110C, 110M, 110Y, and 110K, and developer units 130C, 130M, 130Y, and 130B corresponding to toner colors. The photoreceptor drums 120C, 120M, 120Y, and 120K are adjacent to a transfer belt 140. The transfer belt 140 is circulated by driving rollers 150 driven at a predetermined speed. One of the driving rollers 150 faces a transfer roller 160, with the transfer belt 140 passing between them. Sheets of paper S are fed in the gap between the transfer roller 160 and the transfer belt 140.
A process of forming a color image using the color image forming apparatus having the above-described structure will be described.
Light corresponding to a cyan image is scanned onto the photoreceptor drum 120C by the LSU 110C to form a latent electrostatic image. Cyan toner C included in the developer unit 130C sticks to the latent electrostatic image, and thus a cyan toner image is formed on the photoreceptor drum 120C and transferred to the transfer belt 140. After a predetermined period of time elapses from the time when the cyan image is exposed, the LSU 110M scans light corresponding to a magenta image onto the photoreceptor drum 120M to form a latent electrostatic image. Magenta toner M included in the developer unit 130M sticks to the latent electrostatic image, and thus a magenta toner image is formed on the photoreceptor drum 120M and transferred to the transfer belt 140. Here, the exposing timings of the LSU 110C and 110M are controlled to accurately overlap the cyan toner image and the magenta toner image transferred to the transfer belt 140. Yellow and black toner images are also transferred to the transfer belt 140 using the above-described method, and thus a multi-color toner image is formed on the transfer belt 140. The multi-color toner image is transferred to a sheet of paper S fed between the transfer belt 140 and the transfer roller 160. A fixing unit 170 heats and presses the sheet of paper S to fix and fuse the multi-color toner image to the sheet of paper S. As a result, a multi-color image is completed.
In the above-described color image forming apparatus using the single pass method, a complete color image is formed by only a single rotation of the transfer belt 140. A black-and-white image can also be formed by only a single rotation of the transfer belt 140. In other words, the time required for printing a color image is the same as the time required for printing a black-and-white image. Thus, the color image forming apparatus is mainly used in high-speed printing.
However, if timing for the foregoing exposures is not accurately controlled in consideration of the relative positions of LSUs and photoreceptor drums, multi-color toner images are not accurately overlapped and high-quality color images cannot be formed. Also, since four LSUs and four photoreceptor drums are required, the costs of forming color images increase.
A color image forming apparatus operating in a low-speed mode due to these problems includes a photoreceptor drum and an LSU and uses a multi-pass method in which an exposure process, a development process, and a transfer process are repeated for each of the colors to form a multi-color image. The multi-pass method is classified into a rotary method and a slider method according to the arrangement and switching method of developer units respectively corresponding to colors.
FIG. 2 is a schematic view of a color image forming apparatus using a rotary method. Referring to FIG. 2, the color image forming apparatus includes a photoreceptor drum 220, an LSU 210 which scans light onto the photoreceptor drum 220, a transfer belt 240 which is adjacent to the photoreceptor drum 220, and a turret 280 which rotates. Developer units 230C, 230M, 230Y, and 230K are disposed on the turret 280 such that whenever the turret 280 rotates by an angle of 90° in a counterclockwise direction, the developer units 230C, 230M, 230Y, and 230K sequentially approach the photoreceptor drum 220. The length of the transfer belt 240 is equal to or longer than the maximum length of a sheet of paper S used in the color image forming apparatus.
The operation of the color image forming apparatus having the above-described structure is presented below.
When the developer unit 230C approaches the photoreceptor drum 220 following the rotation of the turret 280, the LSU 210 scans light corresponding to a cyan image onto the photoreceptor drum 220 to form a latent electrostatic image. Cyan toner C included in the developer unit 230C sticks to the latent electrostatic image, and thus a cyan toner image is formed on the photoreceptor 220 and transferred to the transfer belt 240.
After the cyan toner image is completely transferred to the transfer belt 240, the turret 280 rotates again by an angle of 90°, the developer unit 230M approaches the photoreceptor 220, and the LSU 210 scans light corresponding to a magenta image onto the photoreceptor drum 220 to form a latent electrostatic image. Magenta toner M included in the developer unit 230M sticks to the latent electrostatic image, and a magenta toner image is formed on the photoreceptor drum 220 and transferred to the transfer belt 240.
In FIG. 2, timing of the scanning of light corresponding to the magenta image from the LSU 210 is controlled in consideration of the circulation speed of the transfer belt 240 so that the end of the cyan toner image formed on the transfer belt 240 accurately overlaps with the end of the magenta toner image transferred from the photoreceptor drum 220 to the transfer belt 240.
The above-described process is repeated for yellow (Y) and black (K) images. Then, cyan, magenta, yellow, and black toner images are overlapped on the transfer belt 240, and transferred and fixed to a sheet of paper S so that a multi-color image is formed.
FIG. 3 is a schematic view of a color image forming apparatus using a slider method. Referring to FIG. 3, the color image forming apparatus includes developer units 330C, 330M, 330Y, and 330K which are arranged in the direction of movement of a photoreceptor belt 320 and a cam 380 which selectively slides the developer units 330C, 330M, 330Y, and 330K forward and backward in a horizontal direction.
The developer units 330C, 330M, 330Y, and 330K are arranged so that developer rollers 331 are disposed at an initial distance Di from the photoreceptor belt 320. In the color image forming apparatus of FIG. 3, the initial distance Di is greater than a development gap Dg, as shown is FIG. 5, which allows toner on the developer rollers 331 to be transferred to transfer belt 320. Thus, when the developer units 330C, 330M, 330Y, and 330K are maintained at the initial distance Di from the photoreceptor belt 320, toner is not transferred from the developer units 330C, 330M, 330Y, and 330K to the photoreceptor belt 320.
However, when an image is formed, the cam 380 rotates to slide a selected one 330M of the developer units 330C, 330M, 330Y, and 330K toward the photoreceptor belt 320 so that a distance between the selected developer unit 330M and the photoreceptor belt 320 becomes equal to the development gap Dg. Thus, development is possible with only the selected developer unit 330M.
According to the above-described structure, the cam 380 selectively rotates so as to selectively slide sequentially the developer units 330C, 330M, 330Y, and 330K toward the photoreceptor belt 320 so that development is carried out. As a result, cyan, magenta, yellow and black toner images are formed on a transfer belt 340, and transferred and fixed to a sheet of paper S so as to form a multi-color image.
However, in a color image forming apparatus using a multi-pass method as described in FIGS. 2 and 3, unselected developer units are separated from a photoreceptor belt or a photoreceptor drum at a distance greater than the development gap Dg to prevent toner sticking to the unselected developer unit, from being transferred to the photoreceptor drum or the photoreceptor belt and contaminating a multi-color image. The turret 280 should rotate or the cam 380 should operate to slide developer units so that only a selected developer unit is separated by the development gap Dg from the photoreceptor drum or the photoreceptor belt. Thus, an additional driving motor (not shown) is required to operate the turret 280 or the cam 380. Alternatively, if a driving motor (not shown) driving the photoreceptor drum is also used to drive the turret 280 or the cam 380, a complicated switching mechanism is required.
In addition, noise is unavoidable when the turret 280 rotates or the cam 380 operates and the lifespan of a driving system (not shown) may be shortened due to the functional impact with the turret 280 or the cam 380. Also, the impact made by the developing unit reduces the quality of the color images formed.